Why Jupiter's Red Spot Isn't As Great As It Used To Be

Jupiter's Great Red Spot might be, quite literally, the perfect storm: It's a swirling, anti-cyclonic vortex that's big enough to engulf three Earths and has been raging in the atmosphere of the solar system's largest planet for at least 400 years.

Even in a backyard telescope, the Great Red Spot shows up as easily the planet's most prominent feature, sporting "a conspicuous deep red eye embedded in swirling layers of pale yellow, orange and white," as NASA describes it.

We've known about the incredibly shrinking nature of the Great Red Spot for some time, but NASA says that in 2012, amateur astronomers started seeing an accelerated rate of spot shrinkage — by about 580 miles per year. That's an annual reduction equivalent to the distance between Washington, D.C. and Chicago.

"Historic observations as far back as the late 1800s gauged the storm to be as large as 25,500 miles on its long axis," NASA says. "Voyager 1 and Voyager 2 flybys of Jupiter in 1979 measured it to be 14,500 miles across. In 1995, a Hubble photo showed the long axis of the spot at an estimated 13,020 miles across. And in a 2009 photo, it was measured at 11,130 miles across."

Now, the GRS checks in at a relatively diminutive 10,250 miles along its east-west axis.

So, what's going on?

Amy Simon of NASA's Goddard Space Flight Center says that after analyzing images from the Hubble Space Telescope "it is apparent very small eddies are feeding into the storm."

"We hypothesized these may be responsible for the accelerated change by altering the internal dynamics and energy of the Great Red Spot," she says.

Many believe that Italian astronomer Giovanni Cassini was the first to spot the spot in 1665, though he shares credit with Englishman Robert Hooke. (The telescope used by Galileo, who discovered Jupiter's four largest moons in 1609, was apparently not good enough to clearly resolve the GRS, or else the feature did not yet exist.)

The GRS is located in Jupiter's southern hemisphere and rotates counterclockwise once every six (Earth) days.

Universe Today says:

"The spot is prevented from moving north and south across the planet by an eastward jetstream to its south and a very strong westward jetstream to its north. Its position moving east and west changes quite frequently. The spot has 'lapped' the planet at least 10 times since the early 19th century."

Some researchers think that maybe large vortexes such as the GRS survive by swallowing up smaller vortexes, as has frequently been observed on Jupiter.

Researcher Philip Marcus, a fluid dynamicist and planetary scientist at the University of California, Berkeley, says "this does not happen often enough to explain the Red Spot's longevity."

"[Researchers] now find that vertical flows hold the key to the Great Red Spot's longevity: When the storm loses energy, vertical flows move hot and cold gases in and out of the storm, restoring part of the vortex's energy. Their model also predicts radial flows that suck winds from the high-speed jet streams around the Great Red Spot toward the storm's center, helping it last longer."